Recently, in accordance with miniaturization of a semiconductor element, the following method is needed, the method being capable of forming a pattern having a dimension less than an exposure resolution limit in lithography method. As one example of the method, the following method is known, the method including steps of forming sidewall patterns on side surfaces of dummy patterns (core materials) and etching a workpiece film by using the sidewall patterns as a mask. This method is, for example, disclosed in JP-A-2006-303022.
According to conventional methods described in above literature and the like, the dummy patterns located between the sidewall patterns are removed by a wet etching treatment after forming the sidewall patterns, which results in that a microscopical mask composed of the sidewall patterns are formed. Nowadays, it is needed to moreover miniaturize the pattern dimension and improve the dimensional accuracy.